# FORCES AND NEWTONS LAWS by ewghwehws

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```									FORCES AND NEWTON’S
LAWS

2.1-2.2
What is a force?

WHAT IS A FORCE?

A force is any push or pull

WHAT DOES FORCE DO?

A force changes the velocity of an object, because it
changes its speed and/or direction
Why is force hard to define?

• It is something that you take for granted that
exists – although you know about it, you
• Also: there are many forces that are
INVISIBLE
• If you push/pull on an object, you know
where it comes from because you can see it
happening
• But there are some that you can’t see
Examples of “invisible” forces

• Gravity
• Objects falling towards the earth are changing
their speed – can you see what causes them to do
this?
• Friction
• Rolling objects that are pushed come to a stop –
but can you see what causes them to do this?
Enter Newton

• Based on these basic observations, a scientist
named Sir Issac Newton came up with 3 laws
that describe the behaviour of forces
• These are known as NEWTON’S LAWS OF
MOTION
Newton’s first law: ask yourself this

•  Collisions are a good way to start thinking about
Newton’s first law
• When two objects collide – one object applies a
force onto the other
• When a car collides into a wall: what happens
to the car and the wall?
• When a truck collides into a car, what happens
to the truck and the car?
• http://www.consumerreports.org/cro/resour
ces/streaming/crashtestpopups/ToyotaSolar
a/TOYOTA-1.html
What happens?

• Between the wall and the car:
• The wall doesn’t move
• The car rebounds

• Between the truck and the car:
• The truck moves forward
• The car moves forward
Why?

• IS THE CAR APPLYING A FORCE
ONTO THE WALL?
• Yes
• WHY DOESN’T THE WALL MOVE?
• Because the wall is rooted to the ground – it
is hard to move the wall
Why?

• WHY DOES THE TRUCK MOVE
FORWARD EVEN AFTER SLAMMING
INTO THE CAR?
• Because the truck has a heavier mass than
the car – and therefore applies a large force
that can cause the car to move without
stopping itself
In both cases….

• The force that is applied to the object will only
move the object if the force is great enough
• OBJECTS DON’T MOVE OR CHANGE
THEIR MOVEMENT UNLESS THEY ARE
FORCED TO!
• The amount of force required depends on the
object’s:
• Mass
• Friction keeping it in place
What did Newton call this?

• INERTIA: the “desire” of an object to maintain its line
of motion

Only a force great enough to overcome an object’s
INERTIA will cause it to change its direction

That’s why:
• The wall stayed still
• The truck kept moving – the car didn’t apply a great
enough force to overcome its inertia to make it stop
NEWTON’S FIRST LAW: LAW
OF INERTIA

An object at rest tends to stay at rest and an
object in motion tends to stay in motion with
the same speed and in the same direction
unless acted upon by an unbalanced force.
What the heck?

• An object at rest tends   = An object won’t move
to stay at rest             on its own
• and an object in          =something moving
motion tends to stay in     won’t change speed or
motion with the same        direction
speed and in the same
direction                 = unless a force comes
• unless acted upon by        along to change it
an unbalanced force.
What is an unbalanced force?

• Unbalanced = not equal
• For example:
• If one person pushes in one direction on an
object:                    The object moves
because there is only
one force moving in
one direction – IT’S
NOT BALANCED
What happens when forces are
balanced?

• If two people push with the exact same
force but in different directions on the same
object:
The object won’t move
because the two forces
BALANCE EACH
OTHER AND
CANCEL EACH
OTHER OUT!
Newton’s second law

• Physics is still about numbers
• So how do we put numbers into studying forces?
• We have to be able to MEASURE force somehow
– which means numbers
• So: we know force causes an object to move
• How do we calculate movement? (We just studied
this)
• DISPLACEMENT, VELOCITY OR
ACCELERATION
• WHICH ONE IS BEST?
DISPLACEMENT

• Difficult to tell – remember that just
because an object travelled far doesn’t mean
that it was pushed/pulled hard
• You have no idea how long it took to do it
• If you have no idea how much time was
involved – then it could have been going
slow (little force) or fast (more force)
VELOCITY

• Velocity overcomes the problem with
distance – it tells you both time and distance
• But – objects can have a speed even though
nothing is pushing it at the moment
SLIDING OBJECTS
HAVE SPEED BUT
NO APPARENT
FORCE APPLIED TO
THEM – SO SPEED
IS NOT RELIABLE
THAT LEAVES ACCELERATION

• Acceleration is the best way to measure
force
• Acceleration describes changes in speed and
direction – both of which are important to
consider when studying forces
• Slowing down, speeding up or changing
direction ALL REQUIRE FORCES
So what is force dependent on?

• Think about accelerating a truck vs. a car
• WHICH ONE ACCELERATES FASTER?
• A car
• WHICH ONE HAS A STRONGER ENGINE?
• A truck
• LARGER ENGINE: DOES IT EQUAL MORE FORCE
OR LESS FORCE?
• Larger engines generate more force to push a vehicle
• WHY DOES A TRUCK ACCELERATE SLOWER
EVEN THOUGH THE ENGINE IS LARGER?
• Because the truck is heavier
So acceleration of an object depends
on:

• The amount of force
• The mass of the object

Newton’s second law:
The acceleration of an object as produced by a net
force is directly proportional to the magnitude of
the net force, in the same direction as the net
force, and inversely proportional to the mass of
the object.
What the heck?
• The acceleration of an object as   =acceleration created by a net
produced by a net force               force
• is directly proportional to the    =increases if net or unbalanced
magnitude of the net force            force increases, decreases if it
decreases
• in the same direction as the net   =if you push/pull left, object will
force,                                accelerate towards the left -
therefore, an object will
accelerate in the same direction
as net force
• and inversely proportional to      =acceleration increases if mass
the mass of the object.               decreases, decreases if mass
increases
Again….

•   If force increases, acceleration increases
•   The stronger the push, the faster it increases speed
•   If force decreases, acceleration decreases
•   The weaker the push, the slower it increases speed
•   If mass increases, acceleeration decreases
•   The heavier the object, the slower it increases speed
•   If mass decreases, acceleration increases
•   The lighter the object, the faster it increases speed
Now the math..

• Mathematically:

Fnet = ma
• Fnet = net/unbalanced force acting on an
object in N (Newtons)
• m = mass of object in kg
• a = acceleration of object in m/s2
Weight vs. mass

• MASS: the amount of mass in an object measured in g or kg
• WEIGHT: the force of gravity acting on an object
• Every object on earth accelerates towards the earth at 9.8
m/s2
• Therefore, every object’s weight or force of gravity acting
on it is based on Newton’s second law
• Since: F = ma
• And: a = g
• Then force of gravity (Fg):            Fg = mg
The third and funniest law

• We know that objects only move if a force
is applied on them – by the 1st and 2nd law
• But there are some cases where this is
difficult to see
Pushing off a wall?

that pushes off the wall
• What happens?
If the child pushes on the wall
She moves backwards

Does this make sense when you follow
Newton’s second law?
Direction of
acceleration of
No! According to              girl
the 2nd law, the
direction of force
and acceleration has
to be the same
Direction of
force applied
by girl
Based on this fact, Newton came up with the idea
that in order for the girl to be moving backwards –
THE WALL MUST BE PUSHING ON HER!

This is the idea
behind Newton’s            Direction of
3rd law: that              acceleration of
anything that is           girl

pushed pushes
back

Direction of
force applied on
girl by the wall
That’s weird

• Does that sound strange?
• If you think about it – if you apply a force, there has
to be contact between two surfaces
• When two objects touch, force is exchanged
between the two
• Imagine holding two pieces of paper in both hands,
and pushing against each other – they both bend!
• And the force exchanged between the two are equal
Then why doesn’t the wall move?

• This goes back to understanding Newton’s
first law – and also the collision between
the car and the wall
• The girl pushes with a certain amount of
force against the wall – but because the wall
has a lot of inertia, the amount of force is
not great enough to move it
Then why does the girl move?

• The girl has less inertia than the wall
• She is on wheels – which decreases the
amount of force needed to move something
• So even a little force – the amount that she
is pushing off the wall – will cause her to
move
Direction of
acceleration of
Direction of
girl
force applied on
girl by the wall

Direction of
force applied
by girl
Newton’s third law

• For every action, there is an equal and
opposite reaction.
What the heck?

• For every action       • For every push or pull
on an object
• There is an opposite   • The object
and equal reaction       pushes/pulls back in
the opposite direction
on whatever it was
that pushed/pulled on
it
Newton’s      3 rd   law explains why…

• You don’t fall through the floor when you stand on
it!
• Think about it: when you stand on something, you
are essentially pushing down on it because gravity
pulls down on you, and that pushes on the floor
• The reason why you don’t push through the floor is
because the floor pushes back on you
• Since the forces are balanced – you stay on the
ground
If force of gravity is greater than the force of the floor…
If the force of the floor is greater than the force of gravity…

If both are equal…the boy stays on the ground!

Force of floor
pushing back
up on boy

Force of gravity pushing on floor
Normal force

• Normal force is the force that a surface
pushes back on an object that bears weight
on it
• For example: your textbook sitting on your
desk pushes down on your desk, and the
desk pushes back on the book
• The force pushing back on the book is
known as the normal force (FN)
Communicating forces

• Free body diagrams are used to show all the
forces acting on an object
• You’ve seen how they have worked already
in describing the boy on the ground or the
girl pushing on the wall

• The object you are drawing is represented as a box
• Arrows are used to represent each force on the box
• The tail end of the arrows attach to the box
• The arrows move in the direction of the force acting
on the object
• Don’t forget to add forces that you can’t see like
friction, gravity, and normal force
Draw a free body diagram of the forces on the
cart

Normal force
on cart

Force of man
Force of                     pushing cart
friction on
cart

Notice that you don’t
have to worry about              Force of
drawing the man in the           gravity on
free body diagram                cart
Draw the free body diagram of the
forces on the garbage bag
Force applied by the boy
Notice: since the object is hanging       holding the bag
and is not touching the floor             (TENSION)
there is no normal force since
there is no surface contacting the bag!

Force of gravity on the bag

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